In printing systems, sheets are generally transported by a transport belt under the print station. For accuracy reasons, a metal transport belt may be preferred. At some point, the sheets need to be separated from the transport belt. This separation is generally performed at a curve where the transport belt changes direction.
In particular for paper sheets which have just received ink at the print station, the front end of the sheet may become curled as a result of the moisture of the ink. A radius of the curl may be quite small. The curl radius can be so small that the sheet separation becomes unreliable. For this reason, sheet separation systems have been developed which use a local blow zone where overpressure is applied on the underside of the sheets to separate the front end of the sheet from the transport belt and onto a paper guide. The blow units are generally positioned directly upstream of a guide roller where the transport belt changes direction. A paper guide for guiding the separated sheet away from the transport belt is generally positioned just downstream of the roller, more in particular just downstream of the highest point of the roller.
However, a metal transport belt needs to run over a curve having a relatively large radius, because otherwise excessive fatigue would occur in the metal transport belt, resulting in a shorter life span of the metal belt. Therefore a relatively large roller is required at the curved section.
It was recognized in the present invention that if a blow unit of a separation system would be positioned upstream of the roller for a metal transport belt, a relatively large radius of the roller would result in the blow unit being positioned relatively far from the paper guide, and this would disadvantageously affect the separation process.
WO2012/041726 of the present applicant discloses in FIG. 5 a separation system wherein a blow unit 26 is positioned upstream from a roller 14 and at a relatively large distance from the roller 14. A slight change in transport direction of the transport belt is induced in a slight bend to facilitate the separation. The blow unit is positioned just downstream of the small bend. The delivery station 101 is positioned downstream from the highest point of the roller 14, which is the usual position for the delivery station. It was recognized in the present invention that a gap exists between the blow zone 26 and the delivery station 101. The gap is quite large and it was recognized that this large gap disadvantageously affects the reliability of the separation process.
US2008/0001347 discloses a printing system having a sheet separation system. Reference is made to FIG. 3. A blowing unit 64 is located upstream of a guide roller 44 and applies a local overpressure to the underside of the sheets. A further blow unit 38 is located inside the guide roller to separate the sheets from the guide roller close to the paper guide. It was recognized in the present invention that a blow unit inside a guide roller is complex and that it is difficult to obtain a high quality of separation. This is due to the fact that it is very difficult to apply both underpressure and overpressure through the roller, wherein the underpressure is applied just upstream from the region where overpressure. The complexity of this system is also disadvantageous.
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